Seasonal affective disorder (SAD) can be defined as a “recurring depression with seasonal onset and remission.” 1 The Diagnostic and Statistical Manual (DSM-IV-TR) describes it as a “specifier” or a subtype that can occur within any major depressive disorder, bipolar I disorder, and bipolar II disorder.2 There are no planned major changes to the diagnosis of SAD in the upcoming DSM-5.3

Two seasonal patterns of SAD have been described: fall-onset and summer-onset SAD. The fall-onset type, also known as “winter depression,” or “winter blues,” is the most recognized, with symptoms beginning in late fall to early winter and remitting during summer months.1


In any given year, about 5% of Americans experience symptoms of SAD, lasting approximately 40% of the year.3 There is a 4:1 ratio of SAD in women, compared to men.3 Although remission usually occurs spontaneously with the change of the seasons, SAD is nevertheless considered a serious mental health problem, due to its recurrence, duration, impact on quality of life,3 and economic consequences, both individual and societal.4,5

Etiology and Pathophysiology

There are several putative biological mechanisms underlying SAD.3 Most commonly accepted is the phase shift hypothesis (ie, a phase delay or advance in the circadian rhythms associated with the sleep/wake cycle during the winter).6 Other possible mechanisms include retinal sensitivity to light, neurotransmitter dysfunction, genetic variations affecting circadian rhythms, and serotonin levels.3 Since SAD is a complex disorder, it likely is caused by a combination of these factors.3

Presenting Symptoms and Diagnostic Workup

DSM-IV-TR criteria for seasonal pattern specifier include:

  • Regular temporal relationship between onset of major depressive episodes at a particular time of year, unrelated to psychosocial stressors
  • Full remissions also occurring at characteristic time of year
  • Two major depressive episodes in the last two years and no nonseasonal episodes in the same period
  • Seasonal major depressive episodes that substantially outnumber nonseasonal episodes over an individual’s lifetime2

Assessment tools may be useful in screening patients for SAD. The Seasonal Pattern Assessment Questionnaire (SPAQ) has been the most widely studied screening tool,7 but the Seasonal Health Questionnaire (SHQ) may have higher specificity and sensitivity than the SPAQ.7,8

The differential diagnosis is extremely important, as there may be considerable overlap between SAD and other conditions, including major depression, generalized anxiety disorder (GAD), bipolar disorder, panic disorder, bulimia nervosa, late luteal phase dysphoric disorder, chronic fatigue syndrome, and attention-deficit/hyperactivity disorder.7 A pattern of seasonal alcohol use may also be present.7